SHEBA ISFF Flux-PAM Project Report

5.0 Barometers

Atmospheric pressure was measured with Vaisala PTB 220B digital barometers located inside the large insulated boxes containing

J. Militzer

the thermoelectric generators and the station data system. The total accuracy of these sensors, including non-linearity, hysteresis, drift, calibration uncertainty etc., is specified by Vaisala to be ±0.3 mb. In order to minimize dynamic pressure errors, a Nishiyama-Bedard quad-disk pressure port (shown at left, coated with rime) was mounted on the upper surface or lid of the insulated box and connected to the barometer with a flexible hose. This port is expected to reduce dynamic pressure errors to less than 0.1 mb. No problems were encountered with the barometers during the SHEBA field program.

6.0 Hygrothermometers

Atmospheric temperature and relative humidity were measured with NCAR hygrothermometers (TRH) consisting of a Vaisala Humitter 50Y sensor interfaced to NCAR microprocessor electronics and enclosed in a mechanically-aspirated NCAR radiation shield. The Vaisala 50Y sensors employ a platinum-resistance temperature (PRT) transducer and an Intercap capacitive humidity transducer. Since the 50Y PRT has a lower limit of -40 °C, it was supplemented for the SHEBA project with a thermistor calibrated over the range -55 °C to 10 °C. The temperature and humidity sensors were calibrated prior to the SHEBA field project in the NCAR Sensor Calibration Laboratory and the derived calibration coefficients were entered into EPROM memory in the individual TRH electronics. The NCAR hygrothermometer outputs a serial message containing fully-calibrated temperature(s) and relative humidity (with respect to water) data.

The TRH electronics were also used to ingest data from the electronic magnetometer compass. Since the compass data are necessary for determination of wind direction, they are discussed in the section of the report devoted to the sonic anemometers.

Temperature and humidity calibrations were performed at NCAR both before and after the SHEBA field project. The post-project temperature calibrations were nominally within 0.1 °C of the pre-project calibrations for both the platinum resistance and thermistor transducers. During the post-project humidity calibrations, the outputs from four of the 50Y sensors were less than the pre-project calibrations by 2-4 %RH, while a fifth sensor (TRH006) was less by 4-8 %RH. The cause of the departure of TRH006 from the other NCAR 50Ys is not known. Additional post-project laboratory tests were performed to determine the performance of the 50Y sensors near the frost point at low temperatures. The results of those tests are found in Appendix A.

The sign of the humidity calibration shift is also somewhat surprising, because contamination of the capacitive transducers would be expected to increase rather than decrease the sensor output. However, a similar shift (relative to the manufacturer's original calibration) was found in the post-project calibration at NCAR of Vaisala HMP 35 sensors used on the ASFG 20-m tower. A smaller shift of only 1-2 %RH was found for an ASFG hand-held HMP 35 unit, which had much less exposure to the environment over the course of the project. The humidity sensors were calibrated at -15 °C in a Thunder Scientific 2-pressure humidity chamber, with a manufacturer-specified accuracy of ±1 %RH. More details on the calibration procedures and results are available on request from NCAR.

The hygrothermometer radiation shield is composed of two concentric cylinders made of thin-walled brass tubes,

E. Andreas, October 25, 1997

10.75 inches long and with diameters of 1.0 and 1.375 inches. The cylinders are oriented vertically, with the sensor located on their common axis and recessed 7.75 inches above the inlet. The exterior of the shield is painted white and the interior is aspirated with a Conair, 2 inch diameter, 12 VDC fan. After the start of the SHEBA project, we found that the aspiration rate of this shield decreases with increasing wind speed. Measurements are planned to quantify the resulting temperature measurement error as a function of wind speed and radiation load. This information will be included in the project report when it is available. (Following the SHEBA project, the inlet of the radiation shield was modified to alleviate the aspiration problem.) Another potential problem is that frost often covered the shield inlet. The effect of this on the measurements is unknown, but could be investigated by examining the data before and after cleaning of the inlet during station maintenance visits.

Table 6.1 lists the heights of the TRH radiation shield inlets measured relative to the local surface. (On August 24-26, the heights of the hygrothermometer booms were measured at stations 2-4, rather than the inlet heights, and the heights of the inlets were estimated by subtracting 43 cm from those measurements.) The heights of the sonic booms were measured more often than the heights of the hygrothermometer inlets and are also listed in Table 6.1 so that they can be used to estimate the inlet heights when the inlet heights were not measured directly. The variability of the difference of these two heights is perhaps caused by lack of spatial uniformity of the surface.

Table 6.1. Heights of TRH inlets and sonic booms

Station 1				Station 2
Date	TRH	Sonic		Date	TRH	Sonic
97 10 15		2.88m		97 10 15		2.88m
98 04 11		2.26m		98 04 11		2.82m
98 04 17		2.26m		98 04 16		2.87m
98 05 21	1.27m	2.22m		98 05 16	1.81m	2.87m
98 06 12	1.43m	2.39m		98 06 12	2.04m	3.02m
98 06 22	1.61m	2.51m		98 06 23	1.92m	3.03m
98 07 06		2.77m		98 07 07		3.12m
98 07 21		2.87m		98 07 23		3.24m
98 08 03		2.92m		98 07 30		3.23m
98 08 29	1.87m	2.97m		98 08 26	2.18m	3.28m
98 09 16	1.84m	2.90m		98 09 16	2.30m	3.26m
98 09 30	1.69m	2.73m

Station 3				Station 4
Date	TRH	Sonic		Date	TRH	Sonic
97 10 15		2.87m		97 10 23	1.73m	2.90m
98 04 22		2.46m		98 04 11		2.75m
98 05 15	1.43m	2.34m		98 04 20		2.62m
98 06 18	1.71m	2.63m		98 05 16	1.88m	2.84m
98 06 29	1.80m	2.68m		98 06 08	1.87m	2.89m
98 07 09		2.69m		98 06 13	1.86m	2.90m
98 07 30		2.97m		98 07 29		2.93m
98 08 10		2.97m		98 08 26	1.87m	2.95m
98 08 24	1.94m	2.93m		98 09 08	1.87m	2.93m
98 09 20	1.90m	2.92m		98 09 20	1.96m	2.92m
				98 09 30	1.82m	2.94m

Andreas and Claffey have provided 1 hour estimates of TRH and sonic height above snow, based on the above measurements and snowline data from Don Perovich's group. These estimates are documented in an appendix to this report.

Six NCAR hygrothermometers were used during SHEBA field operations. Table 6.2 lists the history of the TRH serial numbers for each station. Each row represents a change in the hygrothermometer at one of the stations. Also included in the table are the apparent reasons for the changes and a reference to the relevant logbook entry. Only one or two changes appear to be associated with failures of the temperature and humidity sensors. Many of the TRH changes were associated with upgrades or repairs that were implemented by successive swaps among the stations. These included an improvement to the radiation shield mount, installation of sonic heaters, and repair of a beacon. (The sonic heaters and beacon were controlled by the TRH microprocessor.) The TRH aspiration fan occasionally attracted the attention of a passing polar bear and thus two changes were needed to repair bear damage. Note that TRH001 saw limited use before failing and did not receive a post-project calibration.

Table 6.2. TRH serial numbers at each station

Date	GMT	1	2	3	4	Comment	Logbook #
		0003	0004	0005	0006	installed
97 10 28	18:50	0003	0004	0005	0007	improve mount	93
97 10 28	23:35	0003	0006	0005	0007	improve mount	93
97 10 29	19:20	0004	0006	0005	0007	improve mount	97
97 10 30	00:50	0004	0006	0003	0007	improve mount	98
97 12 24	21:55	0004		0006	0007	suspicious RH?	170
97 12 27	20:30	0004	0005	0006	0007	bear repair	125,155,173
97 12 31	23:35	0004	0005	0006	0003	test	180
98 01 08	06:10	0004	0005	0006	0007	sonic heaters	195
98 01 13	20:35	0004	0005	0007		sonic heaters	213
98 01 14	03:15	0004	0005	0007	0006	sonic heaters	212
98 01 15	19:55	0006	0005	0007		sonic heaters	217
98 01 21	00:40	0006	0005	0007	0003	sonic heaters	225
98 02 03	22:15		0005	0007	0006	repair beacon	243
98 02 09	00:40	0003		0007	0006	repair beacon	242
98 02 26	01:15	0003		0007	0005	test?	280
98 03 31	01:05	0003		0001	0005	bear repair	344
98 04 06	22:35	0003	0006	0001	0005	resurrected stn 2	362
98 05 11	22:45	0003	0006	0001	0007	check spare	461
98 05 25	22:20	0003	0006	0005	0007	bad RH	485

At the start of the project, there was spiking at a rate of roughly once per 2 hours on the data output by the NCAR hygrothermometer, including the data from the Vaisala 50Y, the supplementary thermistor, and the electronic compass. The cause was traced to a spurious character that intermittently occurred in the serial output message of the hygrothermometer, and this was fixed by modifying the EVE station software configuration to ignore the spurious character. The new configuration was downloaded at station 4 on October 23, at station 1 on October 24, and at stations 2 and 3 on October 26. These spikes were removed during post-project data processing by applying a 1-hour median filter to the hygrothermometer data during this period. Note that this filter also suppresses valid maxima or minima within the 1-hour window in the same manner as erroneous spikes, reducing the variance of the time series.

During SHEBA field operations, it was noted that the 50Y PRT was usually from 0 to 0.2 °C warmer than the thermistor, although the laboratory calibration of both transducers is accurate to better than 0.1 °C. The PRT is the standard Vaisala 50Y temperature sensor and is enclosed, along with the humidity sensor, in a porous shield designed to protect the humidity sensor from contamination. The thermistor was mounted external to the porous shield. We suspect that the PRT was not dissipating its self-heating as effectively as the thermistor sensor, because the porous shield restricts the heat exchange with the ambient air. This explanation is supported by a series of simple tests carried out in early March 1998: when the porous shield was removed, the temperature of the PRT was no longer higher than that of the thermistor, and when it was replaced, the PRT was again warmer than the thermistor. Thus the thermistor data might be expected to better represent the ambient air temperature, but the 50Y PRT data should be used (above -35 °C) to convert the 50Y relative humidity measurements to ambient mixing ratio. The hour-average air temperatures calculated during post-project processing of the data are the PRT temperature above -35 °C (as determined by the PRT) and the thermistor temperature below.